1. Field of the Invention
The present invention relates to filtration systems by which liquids or gases may be clarified in order to separate suspended constituents of a fluid from a filtrate and, more particularly, to filtration systems by which a liquid or gas may be cleansed of suspended particles by a filter medium, which transmits the fluid and captures the particles.
2. The Prior Art
In conventional depth filtration systems, such as a sand filter system, the upstream surface of the filter medium often becomes clogged by large particles. When an impermeable layer is thus formed on the upstream surface, the pressure drop across this impermeable layer increases. As a result, the filtration rate decreases as the large particles on the surface increase, and finally filtration almost stops so that the filter medium must be cleaned or replaced before it has been permeated by the smaller particles. In certain systems, the surface of the filter medium becomes so rapidly coated with an impermeable layer of large particles that a scraper or the like must be operated continuously in order to keep the upstream surface clear. In other words, the upstream surface of the medium typically becomes clogged while a major portion of the interior of the filter medium remains in relatively clean condition.
When the filter medium is made of flexible foam material, upstream pressure forces the impermeable layer downstream-ward, deforming the filter medium in the direction of the flow. When an impermeable layer is formed on the upstream surface, the fluid tries to find a way to flow and seeps between the filter medium and the container walls. This results in the deposition of larger particles on the sides of the filter medium as well. In consequence a small clearance between the filter medium and the container walls is caused. Then the fluid seeps more deeply downstream, so that deformation becomes greater. In addition to the impermeable layer, compression of the flexible foam by deformation suppresses the filtration rate.